The present application is a U.S. national application of PCT/IL99/00520, filed Sep. 30, 1999.
The invention relates to piezoelectric motors and in particular to methods for powering piezoelectric motors using resonant circuits.
Generally, a piezoelectric micromotor is driven with a high voltage AC driving circuit that applies an alternating polarity voltage difference between at least one first electrode and at least one second electrode comprised in the piezoelectric micromotor. The frequency of the AC voltage difference applied to the electrodes is close to a desired frequency of vibration of the piezoelectric motor. To assure proper operation of the motor, the power supply is electrically matched to electrical characteristics of the motor so that power is efficiently transmitted to the motor at the desired frequency of vibration. The at least one first electrode, hereinafter referred to as a first xe2x80x9cdriving electrodexe2x80x9d, and at least one second electrode, hereinafter referred to as a second xe2x80x9cdriving electrodexe2x80x9d, define a xe2x80x9cdriving setxe2x80x9d of electrodes of the piezoelectric motor.
Often a piezoelectric motor comprises more than one driving set of first and second driving electrodes. Different driving sets of first and second driving electrodes are electrified to excite different desired vibration modes in the piezoelectric motor. Electrodes that are electrified by direct connection to a driving circuit while exciting a desired vibration mode are said to be active electrodes and a driving set to which the electrodes belong is said to be an active driving set. Electrodes that are not electrified by direct connection to the driving circuit while exciting a particular vibration mode and the driving sets to which they belong are said to be passive. Passive electrodes are either floating or grounded.
Transmission of power to a desired vibration mode of the piezoelectric motor is generally sensitive to changes in stray capacitance between passive electrodes and ground and changes in capacitance between conducting wires, hereinafter referred to as xe2x80x9cdriving linesxe2x80x9d that connect the driving circuit to the piezoelectric motor. Hereinafter, stray capacitance to ground and capacitance between driving lines are referred to generically as stray capacitance. Changes in stray capacitance generate mismatches between desired resonant vibration frequencies of the motor and frequencies at which power is efficiently transmitted from the driving circuit to the motor. These mismatches can substantially degrade the performance of the piezoelectric motor.
In particular changes in stray capacitance are caused by changes in the lengths the driving lines used to connect the driving circuit to the motor. For example, assume that the driving circuit is matched to a resonant frequency of the piezoelectric motor and that the driving circuit is connected to the piezoelectric motor by driving lines two meters long. If it is required to increase the length of the driving lines to six meters, the increased capacitance between the driving lines changes the resonant frequency of the load that the driving circuit drives and generates a mismatch between the driving circuit and the piezoelectric motor.
An aspect of some preferred embodiments of the present invention relates to providing a piezoelectric motor whose operation is less susceptible than is the operation of prior art piezoelectric motors to the effects of changes in stray capacitance, and in particular to changes in stray capacitance caused by changes in lengths of driving lines that connect the piezoelectric motor to a driving circuit.
In preferred embodiments of the present invention, first and second driving electrodes of a passive driving set of electrodes in the piezoelectric motor are connected in parallel with an impedance substantially smaller than impedance between them resulting from stray capacitive coupling. As a result, mismatches between a frequency at which the driving circuit supplies power to the piezoelectric motor and a desired resonant vibration frequency of the motor caused by changes in stray capacitance, are substantialy moderated.
In some preferred embodiments of the present invention the first and second driving electrodes of the passive driving set are connected by a capacitor. The capacitance of the capacitor is preferably substantially larger than the capacitance generated by any stray capacitive coupling of the first and second electrodes. The connected capacitor, hereinafter referred to as a xe2x80x9cmoderating capacitorxe2x80x9d, is preferably permanently connected between the first and second driving electrodes and is connected between them when they are active and when they are passive. The capacitance of a moderating capacitor while preferably substantially larger than the capacitance of any stray capacitive coupling of its driving set of electrodes, is preferably chosen small enough so that sufficient power reaches the piezoelectric motor when the driving set is electrified by an appropriate AC power supply to excite vibrations in the piezoelectric motor.
In some preferred embodiments of the present invention the impedance between the first and second driving electrodes is reduced to substantially zero by short-circuiting the electrodes for a non-active driving set. The short-circuit is removed when the electrodes are active and used to excite a desired vibration in the motor.
There is therefore provided in accordance with a preferred embodiment of the present invention a method for exciting vibrations in a piezoelectric motor having a plurality of electrode sets, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibrations in the piezoelectric motor, the method comprising; coupling an AC driving circuit to the at least one first electrode and at least one second electrode of a first electrode set; electrically connecting the at least one first electrode to the at least one second electrode of a second set of electrodes with a non-zero impedance that is substantially less than an impedance between them resulting from stray capacitive coupling; and energizing the driving circuit to apply an AC voltage difference between the at least one first electrode and at least one second electrode of the first set of electrodes to excite the vibrations.
Preferably eclectically connecting the at least one first electrode to the at least one second electrode of the second set of electrodes comprises connecting them with a first capacitor having a capacitance substantially larger than a capacitance between them resulting from stray capacitive coupling.
Preferably connecting a first capacitor comprises closing a switch, which switch is operable to be open or closed to respectively disconnect the first capacitor from the electrodes and connect the first capacitor to the electrodes.
Coupling an AC driving circuit to the at least one first electrode and at least one second electrode of the first electrode set preferably comprises opening a switch, which switch is operable to be open or closed to respectively disconnect a second capacitor from between the electrodes and connect the second capacitor between the electrodes.
In some preferred embodiments of the present invention the method comprises connecting the at least one first electrode to the at least one second electrode of the first electrode set with a second capacitor having a capacitance substantially larger than a capacitance between them resulting from stray capacitive coupling.
Preferably, the first and second capacitors are connected permanently between their respective at least one first and at least one second electrodes.
Additionally or alternatively, the first and second capacitors preferably have substantially the same capacitance.
There is further provided in accordance with a preferred embodiment of the present invention, a method for exciting vibrations in a piezoelectric motor having a plurality of electrode sets, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibrations in the piezoelectric motor, the method comprising; coupling a resonant AC driving circuit to the at least one first electrode and at least one second electrode of a first electrode set; short-circuiting the at least one first electrode to the at least one second electrode of a second electrode set; and energizing the resonant AC driving circuit to apply an AC voltage difference between the at least one first electrode and at least one second electrode of the first set of electrodes to excite the vibrations.
Preferably, short-circuiting the electrodes comprises closing a switch operable to be open and closed to respectively disconnect from between the electrodes and connect between the electrodes a substantially zero impedance.
Preferably, coupling a resonant AC driving circuit to the at least one first electrode and at least one second electrode of the first electrode set comprises opening a switch, which switch is operable to be open and closed to respectively disconnect from between the electrodes and connect between them a substantially zero impedance.
There is further provided in accordance with a preferred embodiment of the present invention, a piezoelectric motor comprising: a plurality of sets of electrodes, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibration modes in the piezoelectric motor; and a capacitor for each set of electrodes that connects the at least one first electrode to the at least one second electrode of the set of electrodes wherein the capacitor has a capacitance substantially larger than a stray capacitance between the at least one first and at least one second electrode.
Preferably the capacitor for each set of electrodes has a capacitance substantially greater than the capacitance between the at least one first electrode and the at least one second electrode of the set of electrodes.
Additionally or alternatively, the capacitor for each set of electrodes is preferably permanently connected between the at least one first electrode and at least one second electrode of the set of electrodes.
In some preferred embodiments of the present invention., the piezoelectric motor comprises a switch for each set of electrodes operable to be open and closed to respectively disconnect the capacitor from between the electrodes and connect the capacitor between the electrodes.
Additionally or alternatively all the capacitors preferably have substantially a same capacitance.
There is further provided in accordance with a preferred embodiment of the present invention, a piezoelectric motor comprising: a plurality of sets of electrodes, each set comprising at least one first electrode and at least one second electrode between which AC voltages are applied to excite vibration modes in the piezoelectric motor; and a switch between the first and second at least one electrode, which switch is operable to be open and closed to respectively disconnect from between the electrodes and connect between the electrodes a substantially zero impedance.